Both statements. Starting with if and when are assumptions rather than facts. They could be used interchangeably, but there is a subtle difference. In the first sentence, if is conditional.... It may be useless to buy a boat if you are too busy to use. It, but that doesn't mean you are actually too busy . In the second sentence money is useless when you have no friends...and this applies to situations only when there are no friends, so it is a more definitive statement. If you use 'since' this is a very definitive meaning - you are too busy, or you have no friends whatsoever. It is a very different meaning. Hope this is some help.

This is the "zero conditional," you can search on-line. Yes, "if" and "when" are equivalent.. it's stating a general fact. "Since" would be different, explicitly referring to a specific case.